My Rheem A/C system has a rusted out drain pan on the inside unit. It is dripping water down into the return vent and leaking out. My a/c guy is aware and is looking for the replacement part but is there anything I can do to slow or stop the leak in the meantime.

Try a flat cookie sheet if that will fit under the unit. You don't want water leaking down into your return vent as it will make a mess out of any wood product under there. The a/c guy should be able to get a replacement pan for you shortly I am sure.

You might also be able to repair it if the cancer isn't too bad. Clean up the rust with a wire brush. Paint it with a good rust preventive primer. Get a piece of duct sheet metal and cut to fit, spray the cut edges with a cold galvinizing spray if you can find it. Or just clean and paint it. Lay a good bead of aquarium grade caulk around the edges and pop rivet in place. Should last for a good while. The down side is you have to leave the system off during the rust removal and painting period.